1. Field of the Invention:
The present invention relates generally to magnetic composites comprising a polymer matrix having hard magnetic ferrite particles dispersed therein and, more especially, to magnetic composites exhibiting improved magnetic properties, useful for permanent magnets. The present invention also pertains to methods for producing such magnetic composites.
2. Description of the Prior Art:
The prior art recognizes magnetic composite bodies derived from rubber or plastic matrices charged with ferrite particles. Such magnets or magnetic compositions are disclosed in, for example, British Pat. No. 1,333,174 and No. 1,354,598, among others. As a generalization, these compositions are manufactured by kneading a ferrite powder with a solid polymer and, thence, forming the mixture by extrusion, calendering, or other mechanical working processes.
Magnetic bodies formed in accordance with these prior art techniques have been found superior to sintered ferrite magnets in terms of processibility, as well as flexibility and elasticity, thus rendering these varieties of magnetic bodies particularly suitable in a variety of applications predicated on such qualities. However, these prior art composites have been found to suffer a significant disadvantage inasmuch as they exhibit low magnetic forces precluding their use in applications requiring high magnetic characteristics. Also, thermal stability is observed to be poor, such composites suffering a loss of integrity upon prolonged exposure to temperatures in excess of about 100.degree. C.
The magnetic energy of magnetic composites depends upon (1) the density or amount of ferrite per unit volume of the composite, and (2) the degree of orientation of ferrite particles within the matrix. Typically, where a ferrite powder is incorporated in a polymer, there is encountered an upper limit on the density or packing factor of the ferrite particulate. For example, where natural rubber is employed as the polymer matrix, it has been found impossible, as a practical matter, to incorporate the ferrite particulate in an amount greater than about 9 times the weight of the natural rubber. Additionally, the viscosity of the matrix compound is a significant determinant on the degree of orientation of the ferrite particles within the matrix, the two parameters being inversely proportional.
Accordingly, prior art composites are found to be deficient insofar as both the amount of magnetic ferrite particulate capable of incorporation and the degree of orientation thereof are restricted to limits considered inferior for numerous applications wherein the flexibility of these composites would otherwise prove highly beneficial. Additionally, lack of thermal stability further restricts their utility. Therefore, the need exists to provide magnetic composites, and a method therefor, which overcomes these manifest deficiencies in the prior art.